Conventionally, the number of vehicles including, as a headlight, an HID (High Intensity Discharged) lamp instead of a halogen lamp had been increased, in order to improve the visibility (brightness). However, recently, due to the improvement of luminous efficiency of an LED (light emitting diode), mass production of vehicles including, as the headlight, the LED has been already started. For example, JP2010-143447A (hereinafter, referred to as “document 1”) discloses a lighting device for vehicle including an LED as a headlight.
The lighting device for vehicle in the document 1 includes a first light source unit including a plurality of LEDs, a second light source unit including a plurality of LEDs, and a current-driving unit (DC/DC converter) configured to supply a drive current to each LED. Further, this lighting device for vehicle includes a switching unit (bypass unit), which is connected in parallel with the plurality of LEDs of the second light source unit, and the drive current is supplied to the plurality of LEDs of the second light source unit through off-operation of the switching unit. Further, this lighting device for vehicle includes a controller, which is configured to output a bypass control signal for controlling on/off operation of the switching unit.
This lighting device for vehicle further includes a relay for a low-beam, which is connected between the controller and a vehicle battery, and configured to conduct when a low-beam lamp (the first light source unit) is made lighted. This lighting device for vehicle further includes a relay for a high-beam, which is configured to output, to the controller, a bypass instruction signal for making the switching unit perform the off-operation when a high-beam lamp (the first and second light source units) is made lighted.
Here, in the lighting device for vehicle as the above conventional example, the vehicle battery is used as a power supply. However, a power supply voltage of the vehicle battery fluctuates depending on deterioration of the vehicle battery due to the life thereof, or an increase of consumed current. Furthermore, in the lighting device for vehicle as the above conventional example, a magnitude of the power supply voltage is generally set so as to agree with a condition upon lighting and non-lighting of each beam lamp in order to light the low-beam lamp and high-beam lamp at sufficient illumination. For this reason, in the lighting device for vehicle as the above conventional example, when the power supply voltage of the vehicle battery fluctuates, there is a possibility that it is impossible to normally switch the low-beam and the high beam, namely, lighting states of a load.